1. Field of the Invention
The invention relates to a refractive optical imaging system for imaging an object field arranged in an object surface of the imaging system into an image field arranged in the image surface of the imaging system on a demagnifying imaging scale. The preferred field of application of the invention is projection objectives for microlithography. An imaging system according to the invention can be used as projection objective for microlithography or as imaging subsystem within a projection objective for microlithography.
2. Description of related art
Photolithographic projection objectives with a demagnifying imaging scale (reduction objectives) have been used for several decades for the photolithographic fabrication of semiconductor components and other finely patterned devices. They serve for projecting the pattern of a mask, e.g. of a photomask or of a reticle, onto an article coated with a light-sensitive layer with very high resolution on a demagnifying scale.
Conventional projection systems are designed to image a planar mask onto a flat image field. Accordingly, measures for correcting the field curvature (Petzval correction) are provided in the projection objectives.
To date, purely refractive (dioptric) projection objectives have been predominantly used in optical lithography. These are distinguished by a mechanically relatively simple, centered design that has only a single, unfolded optical axis. Furthermore, use can be made of object fields that are centered on the optical axis and which minimize the photoconductance to be corrected and simplify an adjustment of the objective. The correction of the field curvature lends the objective a characteristic waist structure and gives rise to large overall lengths and large maximum lens diameters, the result of which is to raise the level of the blank mass (mass of the parts of lens material required to produce the lens). Known measures for flattening the image field in the case of refractive optical imaging systems are described in the article entitled “New Lenses for Microlithography” by E. Glatzel in: SPIE vol. 237 (1980), pages 310-320.
A simpler correction of the field curvature, and a possibility of chromatic correction, are reached in the case of catadioptric systems, which have at least one catadioptric objective part with a concave mirror. Here, the Petzval correction (correction for field curvature) is achieved by the curvature of the concave mirror and negative lenses in its vicinity.
The patent U.S. Pat. No. 5,052,763 gives a description of a catadioptric projection objective with an intermediate image in the case of which the image of the object field (intermediate image) produced by a first, catadioptric subsystem is imaged into the image plane with the aid of a second, refractive subsystem. In order to be able to image a planar object into a flat image surface, the Petzval sum of the system is obtained by using the second subsystem to compensate the field curvature produced by the first subsystem, a curved intermediate image surface being produced. The input-end and exit-end subsystems are corrected separately for odd aberrations, such as coma or distortion. Even aberrations such as spherical aberration, astigmatism and field curvature, by contrast, are corrected by compensation between the subsystems. The curved intermediate image is therefore not corrected with regard to these aberrations.
For projection lithography onto curved substrates, the U.S. Pat. No. 6,461,908 B1 proposes using a curved mask whose form is identical to the form of the curved substrate. The curved mask is produced in a contact method. Curvature-conforming imaging of the curved mask onto the curved substrate requires projection objectives with a substantial correction of the field curvature.
The U.S. Pat. No. 5,257,139 discloses a purely reflective reduction objective for extreme ultraviolet radiation (EUV), wherein the object surface and/or the image surface are curved concavely with respect to the projection objective.
In order to be able to produce ever finer structures, there is on the one hand an attempt to enlarge ever further the image-side numerical aperture (NA) of the projection objectives. On the other hand, use is being made of ever shorter wavelengths, in particular UV light with wavelengths of less than 260 nm, for example 248 nm, 193 nm or 157 nm. In this wavelength region, only a few sufficiently transparent materials still remain for producing the optical components, in particular synthetic quartz glass and fluoride crystals, such as calcium fluoride. The fluoride crystal materials are available in suitable quality only to a very restricted extent. Consequently, the aim is objective designs with a low requirement for high-quality lens material.